Method and plant for production of oxygenated hydrocarbons

ABSTRACT

A method for increasing production in an existing processing plant for converting natural gas into a product, wherein the natural gas is first converted into a synthesis gas in a synthesis gas section, the synthesis gas is reacted in a reactor for synthesis of the product, where non-converted synthesis gas and product are separated into two streams, where a product-rich stream is taken out of the process, whilst a product-poor stream is recycled as feed to the reactor together with make-up synthesis gas, and where a portion of the recycle stream is taken out of the recycle loop as a purge gas, where the purge gas is separated into hydrogen-rich and hydrogen-poor streams, where hydrogen-rich streams are introduced into steps in the process where it is desirable to have a supplement of hydrogen, and where the residual thermal value of the hydrogen-poor stream is optionally used for heating before it is discharged. A modified processing plant for carrying out the method is also described.

[0001] The present invention relates to a method for increasingproduction in an existing processing plant and to a processing plantthat has been modified in order to carry out the method.

[0002] Today, in the building of new processing plants, such as, e.g.,plants for the production of methanol from natural gas or other suitablecarbon sources, there is a clear trend towards building plants with anever-increasing production capacity, such as more than 5000 tonnes ofmethanol each day. Thus, production costs are reduced due to economiesof scale.

[0003] In areas where the price of natural gas is low, it is possible toproduce methanol at a cost of as little as about USD 80 per tonne, andthis is allowing methanol to obtain a foothold in a fuel market, i.e.,for fuel cell cars and electricity production.

[0004] For existing processing plants that are too small in relation tocurrent demands, there is a need to find solutions which allow anincrease of total production and thus a reduction in production costsper unit without having to make major costly conversions and refits ofthe existing processing plant.

[0005] It is thus an object of the present invention to provide a methodof increasing production in an existing processing plant withoutnecessitating major, costly conversions of the existing plant. and toprovide a processing plant that has been modified in order to carry outthe present method.

[0006] According to the present invention, this is achieved by means ofa method for increasing production in an existing processing plant forthe conversion of natural gas to a product, wherein the natural gas isfirst converted to a synthesis gas in a synthesis gas section, thesynthesis gas is reacted in a reactor for synthesis of the product,where non-converted synthesis gas and product are separated into twostreams, where a product-rich stream is taken out of the process. whilsta product-poor stream is recycled as feed to the reactor together withmake-up synthesis gas, and where a portion of the recycle stream istaken out of the recycle loop as a purge gas, where the purge gas isseparated into hydrogen-rich and hydrogen-poor streams, wherehydrogen-rich streams are introduced into steps in the process in whichit is desirable to have a supplement of hydrogen, and where the residualthermal value of the hydrogen-poor stream may, if desired, be used forheating before it is discharged.

[0007] It is preferable that the synthesis gas from the synthesis gassection should be fed with a hydrogen-rich stream from the separatedpurge gas, and that this hydrogen-enriched synthesis gas should bepassed through a new once-through reactor for production of the productand through a unit in order to separate a product-rich stream that istaken out, and a product-poor stream that is used as feed in theoriginal reactor.

[0008] It is also preferable that the product-poor stream which is usedas feed for the original reactor should be fed with additional synthesisgas that is produced in a separate secondary synthesis gas line.

[0009] The existin, reactor will preferably be operated virtuallyunchanged.

[0010] It is preferable that the secondary synthesis gas line should bebased on ATR or POX.

[0011] It is also preferable that the product should be methanol ordimethyl ether.

[0012] Also provided is a processing plant for the production of aproduct based on natural gas, where the processing plant comprises asynthesis gas section for producing synthesis gas that mainly consistsof CO, CO₂, H₂ and water, a synthesis section where the product isformed, and a purification section where the product is separated fromunconverted reactants and other substances and purified, whereunconverted reactants that are separated from the product are recycledto the synthesis section, and where a portion of the gas that isrecycled is drawn off in a line in order to prevent build-up of inertgases, where the plant also comprises a separation unit for separatingthe gas that is drawn off in a line into hydrogen-rich and hydrogen-poorfractions, a line for conveying hydrogen-poor fractions to optionalcombustion and lines for leading hydrogen-rich fractions into thenatural gas feed and for recycling to the synthesis sectionrespectively.

[0013] It is preferable that between the synthesis gas section and thesynthesis section there should be provided a once-through reactor forsynthesis of the product, and a separation unit for the separation of aproduct-rich fraction to a line and a product-poor fraction to a lineleading to the synthesis section.

[0014] Furthermore, it is preferable that the processing plant shouldalso comprise a separate secondary synthesis gas line for producing asecondary synthesis gas, and a line for conveying the secondarysynthesis gas as feed for the synthesis section.

[0015] The invention will be described below with the aid an example andthe attached figures, wherein:

[0016]FIG. 1 is a schematic illustration of the structure of atraditional plant for the production of methanol from natural as; and

[0017]FIG. 2 is a schematic illustration of a plant according to thepresent invention.

[0018] Today, the production of methanol is carried out essentially asshown in the schematic diagram in FIG. 1. The processing plant consistsbasically of three sections, a synthesis gas section 2, 4, 8) whereproduction is normally based on natural gas (NG), a synthesis section 14where the actual methanol synthesis takes place, and a distillationsection 17 where the methanol produced is purified.

[0019] The methanol synthesis is takes place by means of the twofollowing reactions:

[0020] 1) CO+2H₂=CH₃OH, or

[0021] 2) CO₂+3H₂=CH₃OH+H₂O

[0022] The synthesis gases, which essentially comprise CO, CO₂ and H₂,in addition to water and non-reacted hydrocarbons, are preparedfollowing one of three different concepts. namely:

[0023] a) conventional steam reforming,

[0024] b) conventional autothermal reforming with a catalyst (ATR) orwithout a catalyst (POX), or

[0025] c) a combination of a) and b).

[0026] Before the natural gas is sent in line 1 to the reformer forsynthesis gas production, sulphur compounds are removed in aconventional manner, and steam is then saturated in and/or addeddirectly to the gas. Saturation can also take place by using a so-called“saturator”. Normally, the gas is also treated in a so-calledpre-reformer 2 before it is sent into the reformer 4, 5, in order toconvert all higher hydrocarbons.

[0027] The following chemical reactions take place during the productionof synthesis gas.

[0028] 3. CH₃+H₂O=CO+3H₂, steam reforming

[0029] 4. CH₄+1.5O₂=CO+2H₂O, partial oxidation

[0030] 5. CO+H₂O=CO₂+H₂, shift reaction

[0031] Reactions 3 and 5 in the reforming reactor are highly endothermicand the heat that is necessary for the reaction can either be suppliedby external firing, as in a steam reformer, or by means of a combinationwith partial oxidation, according to reaction 4, as in an autothermalreformer.

[0032] In a steam reformer (SR), natural gas (NG) (methane) is convertedin a tubular reactor at a high temperature and a relatively lowpressure. A traditional steam reformer consists of a large number ofreactor tubes, usually 100 to 1000, having a tube length of 10-16metres, where each tube has an internal diameter of about 10 cm and anexternal diameter of about 12 cm. This unit may have a length of as muchas 50 metres, a width of more than 10 metres and a height of more than20 metres, with the result that such a reactor will require a relativelylarge space.

[0033] Conventional steam reformers are operated in a pressure range offrom about 15 to 40 bar. The outlet temperature from such a reformer maybe as much as 950° C. The heat required to operate the reaction issupplied by means of external firing or heating and the reformer can betop, bottom or terrace-fired. The heat can also be transferred to thereaction by means of convective heat as in a heat exchange reactor. Theratio of steam to carbon is from 1.6 to 4 and the ratio of H₂ to CO inthe product stream from the reformer is about 3. A typical synthesis gasfrom a conventional steam reformer contains about 3 volume % methane.

[0034] In an autothermal reformer (ATR), synthesis gas production iscarried out mainly by means of reactions 3 and 4, so that the heatnecessary for reaction 3 is generated internally by reaction 4. In anATR, natural gas (methane) is brought together with oxygen-containinggas such as, for example, air, inside a combustion chamber. Thetemperature in the combustion chamber may rise to more than 2000° C.After the combustion, the reactions are brought into equilibrium bymeans of a catalyst before the gases exit the reformer at a temperatureof about 1000° C. The size of an ATR may be a height of 10-20 metres anda diameter of about 4-7 metres.

[0035] An alternative autothermal reformer uses a concept called partialoxidation (POX). A reformer of this kind does not contain a catalyst toexpedite the reactions and therefore as a rule will have largerdimensions than an ATR.

[0036] The reforming of natural gas can also be effected by combinedreforming (CR), where the reformer section consists of an SR and an ATR.A combination of SR and ATR makes it possible to adjust the compositionexiting the reformer section by controlling the admissions of the tworeformers. In CR, SR is operated under milder conditions than in normalSR, i.e., at a slightly lower temperature. This results in a slightlyhigher methane content in the as released from the reformer. Thismethane content is converted in the subsequent ATR. The ratio of carbonto steam in a reformer of this kind is in the range of 1.2 to 2.4, witha ratio of hydrogen to CO in the product gas of well over 2. The optimalstoichiometric number

[0037] (SN═(H₂−CO₂)/(CO₂+CO)) for methanol synthesis is about 2.05.

[0038]FIG. 1 shows a synthesis gas section of the CR type. However, itis not critical what type of synthesis gas section is included in theplant. A plant equipped with a synthesis gas section of the ATR typewill not have SR 4, whereas an plant of the SR type will not have an ATR8 and air separation unit 7 with accompanying line 6.

[0039] After the synthesis gas section 2, 4, 9, the synthesis gas isconveyed in line 9 to a heat exchanger 10 where it is cooled. After theheat exchanger 10, the synthesis gas is conveyed in line 11 to acompressor 12 where it is compressed to the desired pressure in themethanol synthesis section, which is typically about 80 bar.

[0040] The methanol synthesis in the synthesis section takes placeaccording to reaction equations 1 and 2 above, and is an exothermicprocess where conventionally several different types of reactors 14 areused, such as:

[0041] An isothermal tubular reactor with catalyst on the inside ofvertical tubes and boiling water on the outside. The reaction heat willbe removed by partial evaporation of the water.

[0042] Adiabatic fixed bed reactors with cooling between each step

[0043] A fluidised bed reactor

[0044] Adiabatic reactors with cooling by means of a supply of new feedat several levels downwards in the reactor (quench converter system).

[0045] After the reactor 14, the product is fed via a line 15 to a crudemethanol separator 13 that separates the product stream into amethanol-rich stream 34 and a methanol-poor stream 16. The methanol-richstream in line 34 is fed to a conventional methanol purification unit 17that sends methanol out in line 18.

[0046] The methanol-poor product is usually led via a recycle line 16back to the reactor 14. Alternatively, the reactor is a once-throughreactor without recirculation that can be followed by one or moresimilar reactors placed in series.

[0047] A synthesis loop 13, 14, 15 and 16 is shown in FIG. 1. The actualrecycle loop consists of a heat exchanger (“inter exchanger”) (notshown) that preheats the feed to the synthesis reactor and cools theproduction gas, the synthesis reactor(s) 14, a crude methanol separator13 and a system for recovering energy from the exothermic methanolsynthesis reactions (not shown).

[0048] A so-called purge stream is taken out of this recycle loop vialine 19 to prevent the accumulation of inert (non-reacting) gases in therecycle loop. The purge gas in line 19 is often split into a firststream 19 a that is conveyed together with the feed gas in the gassupply 1 and a second stream 19 b that is used as combustion gas forheat-consuming processes in the methanol synthesis, such as in the steamreformer 4, or in another process at the same plant, or is discharged.

[0049] One of the problems of the prior art is that inert gases,primarily nitrogen, in the recycled purge gas will react with otherconstituents of the synthesis gas and be converted to NO_(x) and NH₃ atthe high temperatures that prevail in the reforming plant. According tothe present invention, this is avoided in that the portion of the purgegas that is rich in inert gas is used only as fuel for energy-consumingprocesses, such as the steam reformer 4.

[0050]FIG. 2 illustrates a preferred embodiment of the present inventionwhich has been based upon a traditional existing methanol synthesisplant as described above. A plant having an existing reformer sectionbased on CR has been taken as the basis.

[0051] In this embodiment, purge gas that is taken out of the recycleloop in line 19 is separated in a separation unit 20 into three streams,two hydrogen-rich gas-streams 21, 23 at different pressures, and ahydrogen-poor stream 22. The separation unit 20 is a conventionalhydrogen recovery unit which either works according to the PressureSwing Adsorption (PSA) principle, the membrane principle or is of acryogenic type. The hydrogen-rich fractions preferably have a hydrogencontent of 70 to 100%.

[0052] One of the hydrogen-rich streams 21 is fed into the gas supply 1and is mixed with incoming natural gas. The other hydrogen-rich stream23 is led into the synthesis gas stream in line 11. Alternatively,depending upon the relative pressures in the different parts, thishydrogen-rich stream 23 is led via line 23′ into the synthesis gasstream after the compressor 12 inside a line 24. The hydrogen-poorstream 22 is sent to the burners in the steam reformer 4 as fuel.

[0053] Line 24 conveys the synthesis gas stream from the compressor 12to an extra once-through reactor 25 for the production of methanol. Thissynthesis gas stream is enriched with hydrogen from line 23 or 23′. Thereactor 25 will normally give a yield of about 30% methanol, that is tosay that about 30-35% of the carbon entering the reactor is converted tomethanol. The product stream from the reactor 25 is separated in a crudemethanol separator 33 into a methanol-rich stream 27 and a stream 26that consists primarily of non-reacted synthesis gas and inert gasesfrom the methanol synthesis. Stream 26 is led into a recycle compressor34 in the recycle loop 16 and is introduced into the existing reactor14. Alternatively, if the pressure in line 26 is or is set sufficientlyhigh, stream 26 can be fed as indicated at 26 b past compressor 34 anddirectly to the recycle loop and reactor 14.

[0054] The stoichiometric number, SN, for the synthesis gas in line 9will normally be about 2.06, whilst the SN in line 24 will normally begreater than 2.06 because of the supply of hydrogen-rich gas from line23 or 23′. The methanol-poor stream 26 has a high hydrogen content inrelation to other reactive gases, i.e . . . a high SN that normally willbe greater than 2.10.

[0055] In order to reduce the SN of the gas from line 26, the recycleline 16 is also fed with a synthesis as having a lower SN produced in aseparate secondary synthesis gas line 28, 29, 30, 31, 32. This secondarysynthesis gas line comprises an ATR or POX reactor 30 which receivesoxygen as almost pure O₂, oxygen-enriched air or air, via line 29 fromthe oxygen unit 28 and natural gas from line 31. This new synthesis gasis fed to the recycle line 16 through line 32. This secondary synthesisgas line could also include non-illustrated units such as a compressorand heat exchanger etc. The gas that is introduced into the reactor 30is pre-treated in the same way as the original synthesis gas line, 2, 4,8. Surplus heat in the process can be used in the different separationsteps: if desired, the natural gas feed to ATR or POX 30 can be heatedby using an extra fire heater or heat exchanger using hot synthesis gasfrom ATR or POX 30.

[0056] A combination of preheating and steam reforming in a convectivereformer before ATR or POX 30 is also conceivable.

[0057] Here, it is important that the new separate ATR or POX reactor30, which produces a synthesis as having an SN of less than 2, has acapacity sufficient to allow the SN of the feed to the synthesis reactor14 to be reduced to about 2.06 from an SN of more than 2.10 in the gasin line 26.

[0058] The once-through reactor 25 is usually a once-through reactor ofthe simplest possible type i.e., of the isothermal tubular reactor typeas mentioned above with reference to reactor 14, and will on the outsideof the tubes of the reactor produce more than enough steam at the rightpressure to obtain a favourable steam to carbon ratio inside thereformer. Normally, 30 to 40% of the steam generated on the outside ofthe tubes in the reactor 25 for the cooling thereof will be used as feedfor the new reformer 30 in order to obtain the desired steam/carbonratio therein. The rest of the steam can be used in other steam and/orheat consuming processes, such as, for example, for purifying ! furtherprocessing of the methanol stream. Typically, the methanol-rich productstream contains about 15% water and some ethanol, and distillation isusually carried out in order to obtain pure methanol.

[0059] The conversion in the new reactor 25 will affect the conversionin the existing synthesis reactor 14 so that a reduced conversion inreactor 25 will lead to more unconverted synthesis gas being fed fromreactor 25 to the synthesis reactor 14, with the result that there willbe an increase in the methanol production in 14. The amount of the gasin the recycle line 16 that is taken out as purge gas through line 19and the amount that is recycled in line 16 directly to the synthesisreactor 14 can be varied in order to optimise the system.

EXAMPLES

[0060] The table below shows simulation results for two examples, onefor an existing methanol process according to the outline in FIG. 1, andone for a plant according to the present invention.

[0061] The basis for the simulation is natural as having a methanecontent of about 82% methane. The oxygen feed is varied so that the CH₄slip is about 1.36%. The admissions in the existing reformer section arethe same in both examples.

[0062] The new ATR operates at 35 bar and the steam to carbon ratio(S/C) to the pre-reformer in the new line II is 1.0. The once-throughreactor 25 is positioned after the compressor 12 and has an outletpressure of 80 bar. Example 2 Example 1 New concept Existing forincreased methanol methanol process production Line I NG-rate toexisting line Normalised (%) 100 100 NG-fuel % 100 100 Oxygen tosecondary % 100 110 reformer 8 S/C in feed to reformers % 1.8 1.8 4, 8Temperature in primary reformer 4 Duty % 100 100 Temperature in ° C.1000 1000 secondary reformer 8 Line II NG-rate to ATR line 31 % inrelation to 26 existing plant S/C feed to new pre- 1 reformer Oxygen tonew ATR 30 % in relation to 40 existing plant Inlet temperature in ° C.600 new ATR 30 Outlet temperature in ° C. 1000 new ATR 30 Outletpressure from bar 35 new ATR 30 CH₄ slip from new mol % 1.4 ATR 30Stoichiometric number SN 1. line I from pre- 2.13 reactor SN 2. line I,feed pre- 2.08 reactor SN 4. line II from new 1.68 ATR 30 SN from CR,line 9 2.02 SN 3. line I + II, MUG to 2.06 synthesis loop Synthesis loopRecycle ratio in 4.2 4.2 synthesis loop Total consumption and productionTotal NG to main % 100 process Total O₂-consumption % 100 Totalproduction of % 100 126 crude methanol, I + II (methanol content in thecrude methanol)

[0063] As can be seen from the table above, it is possible in theillustrated example to substantially increase the production of crudemethanol in a process plant for methanol production, without subjectingthe original plant to more stresses than during traditional operations.This is primarily of importance for extensions of existing plants whereit is desirable to use the existing plant to the greatest extentpossible, without having to redimension and rebuild large parts of theexisting plant.

[0064] For the modified plant, it is possible to use surplus materialsor unconverted amounts of a type of reactants in the process bysupplying new amounts of other reactants and thus increase production,thereby rendering it less expensive.

[0065] It is important to note that the layout of the individual modulesand constituents in the exemplified plants may differ from that whichinitially can be understood from the figures. Elements that the skilledperson knows are included or can be included in such plants, such asheat exchangers, compressors, pressure-relief tanks etc., have to someextent been omitted as they are of no significance to the invention.Similarly, the assembly of some of the elements can differ. Thus, someof the elements that are drawn as one unit may consist of severalsimilar or dissimilar elements connected in series and/or in parallelrelation. For instance, the reactor 14 may comprise a plurality ofparallel-connected and/or series-connected reactors.

[0066] The present method and process plant are also useful inconnection with the extension of plants for the production of oxygenatedhydrocarbons other than methanol, such as, for example, dimethyl ether.The construction and mode of operation of a plant for the production ofdimethyl ether are quite similar to those of a plant for methanolproduction, and thus the problems are completely parallel. Although theinvention has been described with reference to a plant for theproduction of methanol, it also comprises other plants as mentionedabove.

1. A method for increasing production in an existing process plant forconverting natural gas to a product, wherein the natural gas is firstconverted into a synthesis gas in a synthesis gas section, the synthesisgas is reacted in a reactor for synthesis of the product, wherenon-converted synthesis gas and product are separated into two streams,where a product-rich stream is taken out of the process, whilst aproduct-poor stream is recycled as feed to the reactor together withmake-up synthesis gas, where a portion of the recycle stream is takenout of the recycle loop an a purge gas, where the purge gas is separatedinto hydrogen-rich and hydrogen-poor streams, where the hydrogen-richstream is introduced into steps in the process where it is desirable tohave a supplement of hydrogen, and where the residual thermal value inthe hydrogen-poor stream optionally is used for heating before it isdischarged, characterised in that the synthesis gas from the synthesisgas section is fed up with a hydrogen-rich stream from the separatedpurge gas and that this hydrogen-enriched synthesis gas is passedthrough a new once-through reactor for production of the product andthrough a unit in order to separate a product-rich stream that is takenout, and a product-poor stream that is used as feed in the originalreactor.
 2. A method according to claim 1, characterised in that theproduct-poor stream that is used as feed for the original reactor is fedwith additional synthesis gas which is produced in a separate secondarysynthesis gas line.
 3. A method according to claim 1 or 2, characterisedin that the existing reactor is operated virtually unchanged.
 4. Amethod according to one or more of the preceding claims, characterisedin that the new synthesis gas line is based on ATR or POX.
 5. A methodaccording to one or more of the preceding claims, characterised in thatthe product is methanol or dimethyl ether.
 6. A processing plant forproducing a product on the basis of natural gas, where the processingplant comprises a synthesis gas section (2, 3, 4, 5, 6, 7, 8) forproducing synthesis gas that essentially consists of CO, CO₂, H₂ andwater, a synthesis section (14) where the product is formed, and apurification section (13, 17) where the product is separated fromunconverted reactants and other substances and is purified, whereunconverted reactants that are separated from the product are recycledto the synthesis section (14), where a portion of the gas that isrecycled is drawn off in a line (19) in order to prevent a build-up ofinert gases, and where the plant comprises a separation unit (20) forseparating the gas that is drawn off in line (19) into hydrogen-rich andhydrogen-poor rations, a line (22) for conveying hydrogen-poor fractionsis to optional combustion, and lines (21, 23) for leading hydrogen-richfractions into the natural gas feed and for recycling to the synthesissection (14) respectively, characterised in that between the synthesisgas section (2, 4, 8) and the synthesis section (14) there is provided aonce-through reactor (25) for synthesis of the product, and a separationunit (33) for separation of a product-rich fraction to a line (27) and aproduct-poor fraction to a line (26, 26 b) that leads to the synthesissection (14).
 7. A processing plant according to claim 6, characterisedin that it also comprises a separate secondary synthesis gas line (29,29, 31, 30) for producing a secondary synthesis gas, and a line (32) tolead the secondary synthesis gas as feed for the synthesis section (14).